scope:

Introduction

Digital transmission network models are hypothetical entities of a defined length and composition for use in the study of digital transmission impairments (e.g. bit errors, jitter and wander, transmission delay, availability, slip, etc.). The diversity of possible network situations requires that individual models can only represent a small portion of typical real entities. However, a limited number of such models (e.g. 2 or 3) together may be sufficiently representative to provide a useful tool upon which studies may be based.

The network models, where applicable, take account of the following features:

a) physically reflect the length of the overall connection with some indication of frequency of occurrence,

b) identify boundaries between switching and transmission elements,

c) give no indication of the means of implementing transmission between switching elements (e.g. metallic, optical, radio media, satellite etc.),

d) describe in detail the user/network access arrangement in the local portion (i.e. customer to local exchange),

e) take account of all possible usages or be independent of them,

f) reflect the use of additional digital processing elements required in particular network configurations (e.g. A-μ converters, digital pads, transcoders, etc.).

This Recommendation makes no statement in respect of the electrical and physical environment in which the network models operate. These aspects are currently the subject of study. In the application of these network models to study of specific digital impairment (e.g. errors) arbitrary judgements may need to be made concerning the significance, in particular, of the electrical environment.